Wonder if he was working from a photo, or if he had a couple of mirrors set up at an angle so he could get the three-quarter view of himself?

I don't know! I have very little information on de Menocal at this point, except that he's most famous for the still lives he began painting in the 1950's. He's in the Smithsonian, but not so far as I can tell the MFA. The back of the watercolor has his birth and death dates and a brief note about Boston.